An optical scanner may advantageously use a contact image sensor (CIS) element or device as the image sensors. The illumination in a contact image sensor may be provided by light-emitting diodes (LEDs) which are very precise components and may therefore be suitable for use in scanners, such as large format scanners. By using LEDs as the illumination source, the CIS will be highly power efficient. However, a problem with light emitted from LEDs is that the efficiency and thereby the light intensity of the LEDs vary with temperature and changes due to aging of the LEDs. This may result in a reduction in the quality of the scanned image. A similar reduction in the quality of a scanned image may also occur when using other light sources than LEDs due to aging and/or temperature variations.
JP60123156 relates to a contact image sensor with a light-emitting diode array used as the illumination source. The contact image sensor comprises a feedback system for controlling the current to the light-emitting diode array. The black/white amplitude of an image signal inputted into an automatic gain control (AGC) fluctuates according to the illumination intensity for a document optical transmissibility of the imaging system, sensitivity of the photoelectric conversion element or the difference between black/white densities of the scanned document itself. When using an illuminating light-emitting diode (LED) array driven by a constant voltage, the emitting luminance changes according to an individual difference of the emitting efficiency, temperature characteristics and aging characteristics of the LED array, and therefore the illumination intensity on the document surface changes, which leads to a change in the black/white amplitude of an image signal.
In order to control the current to the LED, the contact image sensor is equipped with a light quantity detector, a reference voltage generator, a differential amplifier and a conducted current control unit, which operate to keep the light quantity of the illuminating LED array constant via a feedback control.
The invention operates as follows: the light quantity detector detects a portion of an irradiated light emitted from the illuminating LED array and inputs a voltage output corresponding to the illumination density of the document surface into the differential amplifier. The differential amplifier differentially amplifies a voltage corresponding to the reference document illumination density outputted from the reference voltage generator and the output of the light quantity detector, and controls the current conducted to the illuminating LED array via the conducted current control circuit, such that the difference between the two becomes zero. Through such feedback control, the illumination density of the document surface by the irradiated light emitted from the illuminating LED array can be kept constant. Therefore, the AGC does not need to have a function of correcting the illumination density of the document, and it is thus possible to set the dynamic range narrower and avoid a false correction of continuous black signals to white signals.
It remains a problem to improve detection, measurement and compensation for intensity variations from a light source in a scanning element, such as a contact image sensor.